1. Field of the Invention
The present invention relates to a process for producing a vinyl chloride polymer, and relates in particular to a process for producing a vinyl chloride polymer that can control lowering of the heat removal capability of the cooling jacket of a polymerization vessel and lowering of polymer quality caused by an increase of stirring torque in producing a polymer when monomers supplied for polymerization contain a polymerization inhibitor and Fe ions.
2. Description of the Prior Art
In producing a vinyl chloride polymer by polymerizing vinyl chloride or a mixture of vinyl chloride and monomers polymerizable with it in an aqueous medium, the vinyl chloride polymer is usually produced by charging in a polymerization vessel vinyl chloride or a monomer mixture containing vinyl chloride, charging water, a dispersing agent, and a polymerization initiator, and polymerizing vinyl chloride or the monomer mixture for a given time at a given polymerization temperature. If the vinyl chloride or monomer mixture used is transported over a long distance or stored for a long period, as a result of an apparent slurry viscosity increase and stirring torque increase during polymerization, the heat removal capability for reaction heat by the cooling jacket of a polymerization vessel decreases, and controlling the polymerization temperature becomes difficult. Moreover the bulk specific gravity and plasticizer take-up of the obtained polymer decrease, with the problem that the particle size becomes finer. As a countermeasure for this problem, there are methods such as a method in which the amount of the charged water relative to the charged monomer amount is increased, a method wherein the kind of dispersing agent and the amount of adding it are adjusted, and a method in which water is added during polymerization, but were not able to solve these problems sufficiently.
An objective of the present invention is to provide a process for producing a vinyl chloride polymer that can make it easy to remove the reaction heat by a polymerization vessel jacket by stabilizing the stirring torque during polymerization, even if vinyl chloride or a monomer mixture containing it which were transported over a long distance or stored for a long period is polymerized to produce a vinyl chloride polymer without a negative influence on the quality of the polymer.
When vinyl chloride or a monomer mixture containing it is put in a storage tank and transported over a long distance or stored for a long period, a polymerization inhibitor is usually added in the monomers in order to suppress polymerization in the tank. The present inventors, as the result of their zealous investigation, have found out that the cause why this problem occurs when vinyl chloride or a monomer mixture containing it are transported over a long distance or stored for a long period is polymerized is not only the polymerization inhibitor contained in the monomers but that divalent or trivalent Fe ions mix into the monomers from the interior wall of the storage tank due to corrosion etc. during long-term transportation or long-term storing. Furthermore, they have found that this problem can be solved by adding an hydroxycarboxylic acid compound to the polymerization mixture in a specified amount, leading to the present invention.
Thus, the present invention provides as a means for solving the problem a process for producing a vinyl chloride polymer, comprising the steps of:
(A) charging raw materials comprising vinyl chloride or a mixture of vinyl chloride and a monomer or monomers copolymerizable with vinyl chloride and water into a polymerization vessel, said vinyl chloride or the monomer mixture containing at least 0.1 ppm of a polymerization inhibitor and at least 0.01 ppm of Fe ions,
(B) polymerizing the vinyl chloride or the monomer mixture thus charged in an aqueous medium, and
(C) adding a hydroxycarboxylic acid compound in an amount of 50 to 1000 ppm relative to the monomer or monomer mixture to the mixture containing the monomer or monomer mixture in the polymerization vessel, at at least one step of the steps (A) and (B) above.
The process of the present invention is explained hereafter in greater detail. Herein, ppm is the weight measure in the explanation unless specified otherwise.
[Hydroxycarboxylic Acid Compounds]
The hydroxycarboxylic acid compound used in the present invention preferably forms a complex with Fe ions. Fe ions may exist as divalent ones or trivalent ones, or in combination thereof. These Fe ions may exist in a free state or form salts.
The hydroxycarboxylic acid compound is a compound which has a hydroxyl group and a carboxyl group in a molecule, e.g., aliphatic hydroxycarboxylic acids such as glycolic acid, gluconic acid, lactic acid, hydroxyacrylic acid, xcex1-hydroxybutyric acid, glyceric acid, tartronic acid, malic acid, tartaric acid, and citric acid; aromatic hydroxycarboxylic acids such as salicylic acid, m-hydroxybenzoic acid, p-hydroxybenzoic acid, gallic acid, mandelic acid, tropic acid, and alkali metal salts and ammonium salts thereof, among which alkali metal salts and ammonium salts are preferred. Hydroxycarboxylic acid compounds which may be used in the present invention are not limited to these, but any oxycarboxylic acids having an asymmetric carbon atom with the D, L or DL configuration can be used. Also, alkali metal salts and ammonium salts may be in any form of normal salt, acid salt (hydrogen salt), simple salt, and double salt, and a part or the whole may be in a state of salt. These hydroxycarboxylic acid compounds are used as a single kind alone or two kinds or more combined. Among the hydroxycarboxylic acid compounds, especially preferred are alkali metal salts and ammonium salts of malic acid, tartaric acid, and citric acid, and further preferred are ammonium salts thereof.
The amount of the hydroxycarboxylic acid compound to be added in the present invention is 50-1000 ppm relative to the vinyl chloride or monomer mixture containing it, and preferably 100-400 ppm. The timing to add this is not specified, and it is added to or with the mixture of the raw materials at the step (A) and/or to the reaction mixture at the step (B) stated above. Usually it is good to add it to water or vinyl chloride or a monomer mixture containing it when charging the raw materials. There are no restrictions in terms of the adding method etc., either, and it may be in various forms such as solution, aqueous dispersion, and powder, and it is preferable to add it as a solution or aqueous dispersion for ease of handling. If the added amount is too small, the addition effect is not enough, and if the added amount is too large, antistatic properties of the resulting polymer resin is lowered and cost increases, which is not preferable.
[Other Raw Materials]
Monomers and other materials and additives used for polymerization in the present invention may be those usually used for polymerization of vinyl chloride and monomer mixtures containing it in an aqueous medium. Below, they are individually explained.
Monomers
The monomer used as raw material in the present invention is vinyl chloride or a monomer mixture containing vinyl chloride as its main ingredient. The monomer mixture containing vinyl chloride as its main ingredient is a mixture consisting of at least 50 wt. % of vinyl chloride and other monomers copolymerizable with vinyl chloride. The other monomers copolymerizable with vinyl chloride which may be used, for example, include vinyl esters such as vinyl acetate and vinyl propionate; acrylic esters and methacrylic esters such as methyl acrylate and ethyl acrylate; olefins such as ethylene and propylene; maleic anhydride; acrylonitrile; styrene; vinylidene chloride, etc. These can be used as a single kind alone or two kinds or more combined.
Polymerization Inhibitors
When the monomers are transported over a long distance or stored for a long period, dissolution of divalent or trivalent Fe ions occurs in the monomers due to the corrosion of the tank material inner wall etc. containing Fe as its composition. Also, a polymerization inhibitor is added as a stabilizer to the monomers for the purpose of long-distance transportation or long-term storing. As this polymerization inhibitor, for example, included are phenols such as hydroquione, p-methoxyphenol, and t-butylcatechol, and other polymerization inhibitors having a structure of phenol or hydroquinone in the molecules. The present invention can be preferably implemented in the case where the polymerization inhibitor is contained in the monomers in an amount of 0.1 ppm or more, especially 0.1 ppm-50 ppm, and Fe ions are contained in an amount of 0.01 ppm or more, especially 0.01 ppm-20 ppm.
Also, the process of the present invention is preferably implemented in the case where hydroquinone is added as a polymerization inhibitor, especially where the total content of a polymerization inhibitor and Fe ions is 3-30 ppm.
Other than the polymerization inhibitor added for long-distance transportation or long-term storing, the following antioxidants can be added to the monomer or monomer mixture. There is no special restriction on such antioxidants, and they may be those conventionally used in producing a vinyl chloride polymer. Examples include phenol compounds such as 2,2-di(4xe2x80x2-hydroxyphenyl)propane, hydroquinone, p-methoxy phenol, t-butylhydroxyanisol, n-octadecyl-3-(4-hydroxy-3,5-di-t-butylphenyl)propionate, t-butylhydroquinone, 2,5-di-t-butylhydroquinone, 4,4xe2x80x2-butylidene bis(3-methyl-6-t-butylphenol), 3,5-di-t-butyl-4-hydroxytoluene, 2,2xe2x80x2-methylene-bis(4-ethyl-6-t-butylphenol), triethylene glycol bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate], pentaerythrityl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 2,6-di-t-butyl-4-sec-butylphenol, 2,6-di-t-butyl-4-methylphenol, t-butylcatechol, 4,4xe2x80x2-thiobis(6-t-butyl-m-cresol), trophenol, nordihydroguaiaretic acid; semicarbazide derivatives such as semicarbazide, 1-acetyl semicarbazide, 1-chloroacetylsemicarbazide, 1-dichloroacetylsemicarbazide, 1-benzoylsemicarbazide, and semicarbazone; thiocarbazide derivatives such as carbohydrazide, thiosemicarbazide, and thiosemicarbazone; amine compounds such as phenylnaphthylamine, N,Nxe2x80x2-diphenyl-p-phenylene diamine, and 4,4xe2x80x2-bis(dimethylbenzyl)diphenylamine; nitro or nitroso compounds such as nitroanisol, N-nitrosodiphenylamine, nitroaniline, and N-nitrosophenylhydroxyamine aluminum salt; phosphorus compounds such as triphenylphosphite, diphenylisodecylphosphite, phenyldiisodecylphosphite, 4,4xe2x80x2-butylidene-bis(3-methyl-6-t-butylphenyl-di-tridecyl)phosphite, cyclic pentane tetraylbis octadecyl phosphate), tris(nonylphenyl)phosphite, and tris(dinonylphenyl)phosphite; unsaturated hydrocarbon compounds such as styrene, 1,3-hexadiene, and methylstyrene; sulfur compounds such as dilauryl thiodipropionate, dimyristyl thiodipropionate, distearyl thiodipropionate, dodecylmercaptane, and 1,3-diphenyl-2-thiourea. Among these compounds, 3,5-di-t-butyl-4-hydroxytoluene, triethylene glycol-bis-[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate], t-butyl hydroxyanisol, t-butylhydroquinone, 2,6-di-t-butyl-4-sec-butylphenol, and octadecyl-3-(3,5-di-t-butyl-4-hydroxy phenyl) propionate are preferred, because initial discoloration is small when the obtained polymer is formed in a sheet and because scale adhesion to the polymerization vessel is small. In this instance, the initial discoloration indicates discoloration that appears when a vinyl chloride polymer is formed in a sheet for example.
The antioxidant may be used as a single kind alone or as two kinds or more combined, and is added in an amount of 0.006-0.01 part by weight per 100 parts by weight of charged monomers. Also, the antioxidant may be added at any time between the stage of charging the raw materials and the final stage of polymerization.
Dispersing Agents
As a dispersing agent used in polymerizing vinyl chloride or a monomer mixture containing vinyl chloride, included for example are water-soluble cellulose ethers such as methylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, and hydroxypropylmethylcellulose; water-soluble partially saponified polyvinyl alcohols; acrylic acid polymers; water-soluble polymers such as gelatin; oil-soluble emulsifiers such as sorbitane monolaurate, sorbitane triolate, glycerol tristearate, and ethylene oxide-propylene oxide block copolymer; water-soluble emulsifiers such as polyoxyethylene sorbitane monolaurate, polyoxyethylene glycerol olate, and sodium laurate. These are used as a single kind alone or two kinds or more combined, and 0.02-1 part by weight is added per 100 parts by weight of charged monomers.
Polymerization Initiators
As to the polymerization initiator which may be used in the present invention, there is no restriction thereon, and those used in producing conventional vinyl chloride polymers may be used. For example, included are peroxycarbonate compounds such as diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and diethoxyethyl peroxydiarbonate; peroxyester compounds such as t-butyl peroxypivalate, t-hexyl peroxypivalate, t-butyl peroxyneodecanate, and xcex1-cumyl peroxyneodecanate; peroxides such as acetyl cyclohexyl sulfonyl peroxide, 2,4,4-trimethylpentyl-2-peroxy phenoxy acetate, and 3,5,5-trimethylhexanoyl peroxide; azo compounds such as azobis-2,4-dimethylvaleronitrile, azobis(4-methoxy-2,4-dimethylvaleronitrile); potassium persulfate, ammonium persulfate, and hydrogen peroxide. These are used as a single kind alone or two kinds or more combined, and 0.01-0.2 part by weight is added to 100 parts by weight of charged monomers.
In the process of the present invention, other than the above, a polymerization degree regulator, a chain transfer agent, a gelation improver, an antistatic additive, etc. commonly used for producing vinyl chloride polymers can be used upon necessity. Also, antioxidants can be added before, during, or after polymerization for the purposes of controlling polymerization reaction, preventing degradation of the produced polymer, etc.
Other Polymerization Conditions
Other conditions in polymerization such as the method of charging in a polymerization vessel and ratio of charging of an aqueous medium, vinyl chloride or a monomer mixture containing vinyl chloride, dispersing agents, and a polymerization initiator, the polymerization temperature, etc. may be the same as conventional by used.